Copending application Ser. No. 478,431, filed June 11, 1974, now U.S. Pat. No. 4,006,189 relates to a process for the purification of glyoxal solutions, especially those which have been obtained by the oxidation of acetaldehyde with nitric acid, the raw glyoxal solution, freed of the volatile acids, being neutralized, the glyoxal being transformed to the glyoxal semiacetal with an alcohol having 1 to 3 carbon atoms, with removal of the water, and, after separation of the salts and other impurities, being hydrolyzed again with water at elevated temperature.
The present invention relates to an improved process for the purification of glyoxal and consists in a continuous process whereby technical raw glyoxal solutions are made into acid-free and colorless aqueous glyoxal solutions.
Glyoxal is prepared technically by the oxidation of acetaldehyde with aqueous nitric acid. The raw product thus produced contains, in addition to the unreacted starting substances acetaldehyde and nitric acid, a number of by-products whose complete separation presents considerable difficulty.
The acids involved are acids which are volatile at elevated temperatures, namely acetic acid and formic acid, acids of low volatility, namely glyoxalic acid, glycolic acid and oxalic acid, and also inorganic electrolytes and other impurities which cause a more or less great discoloration of the product.
It is possible to remove most of the volatile acids by distillation and concentration of the solution, but a crude glyoxal is obtained from which the remaining acid cannot be removed by this method.
In older processes, these acids are neutralized with carbonates of a metal of the Second Group, especially calcium carbonate; however, the separation of the calcium salts in solid form is incomplete, so that the solution is additionally contaminated with calcium ions. The glyoxal solutions thus refined have a strong yellow discoloration.
Attempts have also been made to remove the non-volatile acids with ion exchangers, for example in accordance with German Auslegeschrift No. 1,154,081.
If one compares the acid content of a crude glyoxal solution subjected to preliminary purification by distillation with the capacities of an anion exchanger (.about.1.5 val/l) it will be seen that, for the deacidification of a 40% crude glyoxal solution (D 1.27), an equal volume of the ion exchange resin will be required. Then, the fact that the removal of the glyoxal solution from an exhausted anion exchange resin requires a six-fold to ten-fold volume of washing liquid, and an equal volume of washing liquid will again be consumed after the resin is regenerated with caustic soda solution, places a considerable burden on the economy of this method.
The high water consumption will be disadvantageous even in the electrolytic dialysis method described in German Auslegeschrift No. 1,618,281. The authors achieve a glyoxal yield of 80 to 90% at a detention time of 20 hours.
In the method of Russian Pat. No. 168,670, the crude glyoxal and ethanol are used to prepare first the tetraacetal which, after being refined by distillation, is hydrolyzed on acid cation exchangers. However, according to our own experiments and reports given in the literature (J. Am. Chem. Soc. 77, 1285, 1955), the acetal formation is unsatisfactory even with a great excess of alcohol.
German Offenlegungsschrift No. 2,159,975 describes the preparation and use of glyoxal semiacetals and also the use of glyoxal semiacetals as intermediates for the production of acid-free, aqueous glyoxal solutions. In this procedure the starting products are technical glyoxal solutions from which the glyoxal semiacetals are isolated by reaction with alcohols which are not miscible in water, and are purified. In a three-stage hydrolysis with boiling water, however, only 80% can be obtained as hydrate. The solution obtained still contains definite traces of the alcohol put in. The essential disadvantages of the described method is the incomplete separation of the glyoxal from its solution, which has not been achieved even when an excess of the alcohol was used.
In the method of copending application Ser. no. 478,431, now U.S. Pat. No. 4,006,189 technical crude glyoxal solutions, after separation of the volatile acids, are neutralized, the glyoxal is transformed to the glyoxal semiacetal with an alcohol having 1 to 3 carbon atoms with removal of the water, and after separation of the salts and other impurities it is rehydrolyzed with water at elevated temperature.